The release of environmental contaminants from underground storage tanks and associated devices such as piping, fittings, flanges, valves, and pumps by leakage, spills, overfilling, etc. is a serious, pervasive source of environmental pollution and a problem which has been taken up by numerous regulatory agencies.
In the United States, for example, a number of rules and regulations addressing this problem have been promulgated by the Environmental Protection Agency (EPA). Among other things, the EPA standards require that regulated tanks and piping be monitored for the leakage or other release of hazardous substances. This regulation can be complied with by employing a leak detection and monitoring system having the following minimum capabilities for tightness testing of tanks and piping:
______________________________________ Leak Detection Rate 0.10 gallon per hour Probability of Detection .gtoreq.95 percent Probability of a False Alarm .ltoreq.5 percent ______________________________________
Aside from the environmental pollution that results, leaks from containment vessels are undesirable because of the safety hazards and consequent liability the leaked substance may present and/or because of economic factors. For example, as the cost of crude oil increases, the economic penalties appurtenant to leaks of the oil and products refined from it become increasingly severe. Such considerations have led the American Petroleum to recommend a leak detection threshold rate of 0.05 gallons per hours.
A comparable problem exists in aboveground storage systems. Leaks from aboveground tanks, pipes, and other containment vessels cause significant contamination of soil, air, surface water, etc. in the surrounding environment. And, again, no satisfactory system for monitoring such systems and detecting leaks with a satisfactory level of accuracy has been found to currently be available.
A number of patents and other documents of which I am aware disclose leak detection and monitoring systems.
These are:
______________________________________ Patent Number Patentee(s) Issue Date ______________________________________ U.S. PAT. NOS. 3,564,526 Butts 16 February 1971 3,848,765 Durkop 19 November 1974 4,568,925 Butts 4 February 1986 4,602,250 Peace 22 July 1986 4,648,523 Strock 10 March 1987 4,670,847 Furuse 2 June 1987 4,685,327 Sharp 11 August 1987 4,696,186 Sharp 29 September 1987 4,708,015 Sharp 24 November 1987 4,787,772 Wagner 29 November 1988 4,796,676 Hendershot et al. 10 January 1989 4,805,444 Webb 21 February 1989 CANADIAN PATENTS 775,757 Hakka et al. 9 January 1968 775,758 Butts 9 January 1968 978,614 Waikowski et al. 25 November 1975 1,120,131 Butts 16 March 1982 OTHER Jacketed Steel Underground Storage Tanks, Total Containment, Inc., Exton, PA Flexible Inner Tank System, World Enviro Systems, Inc., Shawnee, OK ______________________________________
It is problematical whether any of the systems disclosed in the above-cited documents would meet the above-identified, current EPA standards let alone the more exacting ones that may be enacted by the EPA or other regulatory agencies in the future or dictated by economic considerations such as a rise in the cost of crude oil. With one or two possible exceptions, the previously proposed systems are capable only of gross leak detection, not the sophisticated leak detection and monitoring required by relevant EPA and other regulations.
Those previously disclosed systems which do appear to have a potential for more than the gross detection of leaks--for example, that disclosed in Hendershot et al.--detect leaks by monitoring a negative pressure in the space between a tank and a flexible tank liner. In each instance, however, the monitoring system is inaccurate because it does not take into account the effect of permeation on the measurements made by the system.
The primary containment vessels and liners of underground storage tanks and associated devices are fabricated of materials which are more-or-less permeable to the substances stored in them. Even a modest permeation of one ounce of hazardous substance per 24 hours per square foot of containment vessel surface can result in the tank-liner interspace being filled with the hazardous substance at a significant rate. Consequently, a small leak of the magnitude which is required to be detected by current EPA standards may be masked by a changing interspace pressure attributable to the permeation and will remain undetected. And, in many cases, this problem of permeation masking leaks is complicated by the combination of outward permeation of a contained substance into a tank/liner interspace and by permeation of ground water and/or other substances inwardly to that space. This significantly lessens the probability that, in the heretofore proposed systems, the conditions in the tank/liner interspace will have the constancy needed so that changes in that space indicative of a small leak can be detected.
There are a number of leak detection techniques other than those reported in the above-cited patents and non-patent documents.
The use of double wall piping with leak detection tracers in the annular space between the inner and outer walls has been, and is, fairly widely used in the chemical industry where losses due to leaks are possibly both expensive and dangerous.
Volumetric leak detection systems, also, have been available for many years. The most familiar volumetric leak detection method is the gauge stick used at service stations. In this approach, the operator, by reconciliation of tank volume readings with tank inputs and outputs, attempts to discover if the tank or ancillary equipment (pipes, pumps, or dispensers) is losing fuel to the surrounding environment. The use of this system, mechanized if not electronically calculated, is the basis for an entire industry attempting to discover and verify leaks or their absence. Under the EPA rules for underground storage systems, such a system is accepted under certain conditions. However, all such systems rely upon a conversion of height to volume, the factoring in of temperature gradients, surrounding environmental changes, and so on. The control of so many variables has made the development of accurate and reliable volumetric systems very difficult and has led to EPA regulations banning the use of such systems in a stand alone mode for leak detection.